I've baked in stainless steel bowls but I'm thinking that a bowl that big would take forever to bake and might have overly dry parts because of it. You might be better off stacking several round cakes and carving it.
I don't know if this is an option, but when I have a request for a large baby belly cake, I make a sheet cake and put a regular size belly cake on top of that. It gives you the servings you need without forcing you to buy special pans. You might want to check with your client and see if that'd work for her.